The neurophysiology of behavior is being studied in an Aplysia preparation developed in this laboratory. We have identified neurons which control the parameters of habituation, i.e., response decrement and dishabituation. Habituation is controlled by the central and the peripheral nervous systems, with the central nervous system exerting its action via the peripheral nervous system. Central control is dominant and is adaptable to changes in stimulus conditions. Central control is age-dependent, i.e., absent in young animals, thus development of control and its neural substrates can be studied. Habituation, considered an archetype of learning, involves comparatively simple neural networks in Aplysia, and thus its physiological correlates are amenable to analysis. A neural model has resulted from this work and should aid in achieving the following objectives: 1) To investigate the physiological basis of greater adaptability in the habituation paradigm of central neurons as compared with peripheral neurons. The comparison can reveal properties characteristic of central neurons. 2) To examine electrophysiological and morphological properties of central neurons, involved in habituation, of young and older animals. The development of neuronal properties characteristic of central neurons can be studied. 3) To study changes in synaptic efficacy during habituation. Our results indicate that inhibition is responsible for control of habituation. In young animals the behavior and activity of central neurons reflect the absence of inhibition. 4) To correlate neural coding with parameters of habituation. The parameters appear to be dependent upon spike frequency in single neurons. 5) To investigate the hierarchical relationship between the central and peripheral neurons controlling habituation. The neural circuits in the model may be prototypic of those controlling more complex behaviors.